Downloaded 151 times
Uploaded bym_divya_bharathi
Paper unit i
This document provides an overview of the papermaking process. It discusses the various raw materials used, including various plant fibers. It then covers the pulping process, which separates fibers from wood or plant materials. It describes different pulping methods like mechanical, chemical, and combinations. Next, it discusses bleaching, stock preparation, and the addition of fillers and sizing agents. The document concludes with an overview of dyeing methods for paper.
More Related Content
Paper unit i
- 1.
- 2. INTRODUCTION • Paper isa thin material mainly used for writing upon, printing upon or for packaging. • It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets.
- 3. Raw materials Five differentfiber classifications include: – bast or stem fibers, which are the fibrous bundles in the inner bark of the plant stem running the length of the stem; – leaf fibers, which run the length of leaves; – seed-hair fibers; – core, pith or stick fibers, – Grass and all other plant fibers not included above
- 4. Bast fibers Examples ofbast or stem fibers include; – jute, – flax, – hemp, – kenaf, – ramie, – roselle, – andurena.
- 5. BAST FIBER • • • • • • Obtained fromthe flax plant. Located inside the stem. Only 5% usable for papermaking. Hemp and jute are other forms of bast fibers. Hemp has been used for cigarette and Bible papers. It is used for heavy duty shipping tags and heavy pattern boards.
- 6. Leaf fibers • Leaffibers include; – banana, – sisal, – henequen, – abaca, – pineapple, – cantala, – caroa, – mauritius, and phormium
- 7. Cont… Seed-hair fibers include; –coir, – cotton, – kapok, – and milk weed floss. Center or pith fibers include; – kenaf – and jute.
- 8. Seed Hair Fibers—CottonPlant • Cotton fibers are very expensive. • Used primarily for bank notes, high grade writing papers, maps—anything that requires exceptional strength and durability.
- 9. Grass Fibers • WheatStraw – Absorbs water readily. – Used to make low grade paper. • Bagasse fiber – Made from crushed stalks of sugar cane. – Used as a source of fuel for Sugar Mills. • Esparto Plant – Has very little strength and does not split into fibrils easily.
- 10. Grass Fibers • Kenafplant – Shows great potential for papermaking. – May produce 5 to 7 times more pulp/acre than pine. – Grows to maturity in 120 days compared to 20 years for most trees.
- 11. The remaining fibersinclude; • • • • • roots, leaf segments, flower heads, seed hulls and short stem fiber
- 12. Wood Fibers • 95%of all paper is made from wood fibers. • Deciduous hardwood trees, e.g., Oak, Gum, Maple, Aspen, etc. • Coniferous softwood trees, e.g., Pine, Spruce, Fir, etc. (the softwoods produce a stronger pulp).
- 13.
- 15.
- 16. Cellulose properties • Itis abundant and replenishable • It can be easily harvested and transported to its usage site • Good tensile strength • Great affinity for water • They resist change or degradation by any chemicals bcoz of this unique characteristics.
- 17. Pulping • The objectiveof pulping is to separate the wood into individual fibers. • A pulp mill is a manufacturing facility that converts wood chips or other plant fiber source into a thick fiber board which can be shipped to a paper mill for further processing.
- 18. Pulping methods Three broadclassifications of pulping methods: – Mechanical (groundwood) – Mechanical (refiner) – Chemical (kraft process) – Chemical (sulphite process) – Combination (chemi-mechanical) – Combination (chemi-thermo mechanical)
- 19. Mechanical pulping • Grinding-Stone Groundwood(SGW) – Logs (very occasionally chips) are pressed into a turning stone thus releasing fibers.
- 20. Ground wood • Thebark is removed from the logs. • Then, the cut logs are forced by hydraulic or steam pressure against a revolving grinding stone in the presence of water. • This treatment converts the wood into a pulp consisting of minute particles of both fibrous and non fibrous portions of wood.
- 21. • The nonfibrousmaterials deteriorate when left for some time in contact with air. • This type of pulp is not as strong as chemical pulp. • Groundwood pulp has to be mixed with other pulps, e.g., Newsprint—contains 80% groundwood pulp and 20% chemical pulp.
- 22. Advantages • Low costand high yield • This pulp has high bulk and opacity • Excellent printing, cushioning and ink absorbency • Disadvantages; – Low strength and brightness
- 23. Refining pulp • Refining-RefinerPulp – Chips are fed between 2 disks. One disk is always turning while the other can be fixed or turning.
- 24.
- 25. Cont… • Small cylinderor wood chip is placed between two opposite rotating plastic disc • Chip disintegrates into coarse fibers • Compression and decompression of rotating disc • Heat generated splits the individual fibers.
- 26. Advantages • Refiner mechanicalpulp also utilizes waste chips and low grade wood than ground wood • It has the high opacity and yeild
- 27. Chemical pulping • Itis used to isolate fibers from wood compartment is to remove the lignin • Delignification is done by degrading the lignin molecules, bringing them into solution and removing them by washing • Also a certain amount of carbohydrates (cellulose and hemicelluloses) is lost in this process
- 28. Sulphate process • Thekraft process (or sulphate process) is the dominant chemical pulping method worldwide • Which involves cooking the chips in a solution comprising sodium hydroxide ( NaOH) and sodium sulphide (Na2S), with OH and HS- as the active anions in the cooking process • The hydrogen sulphide is the main delignifying agent and the hydroxide keeps the lignin fragments in solution
- 29. Sulphite process • Thesulphite process involves dissolving lignin with sulphurous acid (H2SO3) and hydrogen sulphite ions (HSO3-) as active anions in the cooking process • More recently devolped pulping methods include the use of organic solvents as ethanol, methanol and peracetic acid (CH3CO3H) for delignification .
- 30. TMP • To makeTMP, wood chips or sawdust are first softened by steam and then subjected, under pressure, to the defibering action of a disk-type refiner. • This causes the fibers to completely separate from each other and suffer less damage than those produced by the conventional groundwood pulp process. • TMP is cleaner and stronger when compared with groundwood pulp.
- 31. Advantages • TMP isstronger than RMP and stone groundwood and it eliminate more costly chemical pulps that are blended with mechanical pulp • It has high yield and high opacity • It has better runnability on the presses.
- 32. CTMP • In thechemi-thermo mechanical pulping, where the wood chips are treated with mild chemicals prior to the refining • This mild chemical pretreatment improves pulp brightness to some extent
- 33. Bleaching technique • Allpulp requires bleaching to remove residual lignin before papermaking process • The purpose of bleaching is to remove stains caused by lignin • There are many different types of bleaching process, involving different chemicals and conditions
- 34. Bleaching agents • Majorbleaching agents – chlorine, – sodium hypochlorite, – hydrogen peroxide, – chlorine-dioxide, – and oxygen
- 35. Cont… • Chlorine gasis one of the different bleaching systems that are used to remove lignin that remains in the pulp. – Chlorine gas is passed into the pulp-water mixture. – The gas reacts with whatever lignin that is left and becomes chlorinated. – The chlorinated lignin is then removed when treated with the caustic soda NaOH (Sodium Hypochlorite)
- 36. Mechanical pulp bleeching •After the refining process, the resulting fibers do not have the brightness required for most paper applications. Bleaching is therefore required. • For the mechanical pulp, hydrogen peroxide, sodium hydrosulphite or formamide sulphuric acid are used for bleeching • Chlorine was widely used in the past but few mills use it today.
- 37. Chemical pulp bleeching •In chemical pulping, the majority of the lignin has been removed in the pulping stage • In general kraft or sulphate process requires more bleeching than sulphite • Sodium hypochlorite or hydrogen peroxide may be used for brightening chemically pulped fibers
- 38. Cont… • Once thepulp has been screened, cleaned and bleached it is ready for the stock preparation
- 39. Stock preparation • Itinvolves many operations, including stock blending, refining, broke management, operation of the several system, and mixing and blending of wetend additives. • All these are important operations in determining the quality of paper and paper machine efficiency.
- 40. Cont… • The pHis controlled and various fillers, such as whitening agents, size and wet strength or dry strength are added • Additional fillers such as clay, calcium carbonate and titanium dioxide to increase opacity
- 41. Fibers & nonfibers • Fiber - Selected for properties & cost • Non-fibrous Additives - Selected to obtain properties not inherent in fiber – Strengthening Agents (Starch) – Fillers (Clay, CaCO3, TiO2, etc) – Internal Sizing Agents (Rosin,etc.) – Coloring agents – Other
- 42. Process • Pulp ispumped through a sequence of tanks that are commonly called chests. • Chests may be either round or rectangular made with special ceramic tile faced reinforced concrete, mild and stainless steels are also used • Pulp slurries are kept agitated in these chests by propeller like agitators near the pump suction at the chest bottom.
- 43. Different levels • Productionlevel – To break down the raw material into individual fibers, – to separate fibers and contaminants, – to separate fibers/solids and water, – to treat the fibers, and – to treat the residual contaminants.
- 44. • Recovery level –where fibers and other solids, and water are recovered from the rejects of the separation processes applied in the production level. Again fibers and contaminants as well as solids and water have to be separated. • Discharge level – for ecological and economic reasons the final rejects are separated into wastewater and residues with high dry content.
- 45. Effect of addinglong fiber
- 46. Effect of refiningon pulp/paper properties
- 47. Pulp Composition • HighHemicellulose content - Easy to refine, strong & dense sheet. • High Lignin - Difficult to refine, and inhibits swelling. • High Carboxylic Acid Content – Causes fiber swelling, and ease in refining • pH - Easier at high pH, but difficult to generalize.
- 48. FILLERS Fillers are appliedto the paper mainly; • To improve the optical properties, such as brightness and opacity • To improve the smoothness of the sheet surface (i. e. decreased roughness, especially after calendering) • To improve the sheet formation by filling the voids between the fiber matrix
- 49. Cont… • To enhanceprintability in the various printing processes due to a more uniform paper surface, higher opacity and better ink receptivity. • The latter resulting in reduced printing ink penetration, wicking and ink strike-through to the opposite side of the sheet
- 50. Cont… • To improvethe dimensional stability of the paper as most fillers remain inert when wetted, unlike the natural fibers usually used in papermaking. • To improve the permanence of the paper (alkaline papermaking, calciumcarbonate (CaCO3) filler)
- 51. Sizing • Sizing makesthe fiber hydrophobic and thus prevents or reduces the penetration of water or other aqueous liquids into the paper. • Sizing prevents the spreading and strike through of ink or printing colors.
- 52. Cont… • Papermaking fibershave a strong tendency to interact with water. • A high absorbency is important for a few paper grades such as toweling and tissue. • Also corrugated medium paper must be “absorbent” to a certain degree to convert properly in the corrugating process.
- 53. Cont… • On theother hand such properties are disadvantageous for many paper grades e. g. liquid packaging, top layer of corrugated board, writing and printing papers and most of the specialty papers. • The water and liquid absorbency can be reduced by the addition of sizing agents to the paper stock and/or by their application to the paper surface.
- 54. Process • Sizing isusually performed in a size press or a film press. • In a size press, the web is passed through a pond of the sizing agent, which is located above a roll nip. • As a result of both capillary action in the pond and the hydraulic pressure in the roll nip, the paper web absorbs the sizing liquor.
- 55. Cont… • The amountof size pick-up and the degree of penetration depend upon the – the concentration and viscosity of the size, – the absorption behavior (porosity, moisture content, temperature, etc.) of the paper web, – and the nip pressure and nip length. – Control of the size pick-up is mainly by variation of the size concentration, but also by variation of the nip pressure.
- 56. Dying • Stock dying •Surface dying • Dip dying
- 57. Stock dying • Thisprocess is called internal dyeing and is the most widely used paper dyeing process. • Because of clean working conditions and the most efficient usage, the dyes are now mostly added continuously and fully automatically into the stock flow.
- 58. • choice ofdye and the fixing and dyeing conditions largely depend on the raw materials used in papermaking (recycled fibers, stone groundwood, TMP, CTMP, unbleached or bleached chemical pulp, type and portion of filler) and on its preparation process • Eg. higher degree of beating of the pulp results in a deeper coloring • Fillers increase the required amount of dyes because they absorb dyes and, at the same time, reduce the coloration
- 59. Cont… • Addition ofdye is determined by a few factors e. g. high consistency dyeing at a stock consistency of 3–4 % (before mixing with white water ahead of the headbox) • Alternatively low consistency dyeing at a stock consistency of 0.5–1.5 % (in front of the mixing pump or pressure screens). • The pH conditions are very important.
- 60. • The additionof aluminum sulfate usually promotes the absorption of dyes and yields less colored waste water and effluent. • In general, there is a trend towards paper production in the neutral or alkaline pH range. These conditions need dyes with a very good affinity to the paper stock in a neutral medium and/or very effective fixatives and retention aids.
- 61. Advantages and disadvantages •Batch addition has the advantage of thorough mixing of the additives with the paper stock and optimal fixation due to longer contact time between the fiber and dye. • The disadvantages are that the time required for color correction and color change is relatively long (loss of productivity).